Passivation of infrared detectors using oxide layer

ABSTRACT

An infrared detector and a method for manufacturing it are disclosed. The infrared detector contains an absorber layer responsive to infrared light, a barrier layer disposed on the absorber layer, a plurality of contact structures disposed on the barrier layer; and an oxide layer disposed above the barrier layer and between the plurality of the contact structures, wherein the oxide layer reduces the dark current in the infrared detector. The method disclosed teaches how to manufacture the infrared detector.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/212,789 filed Mar. 14, 2014, which claims the benefit of U.S.Provisional Application No. 61/793,353, filed on Mar. 15, 2013, theentirety of both of which are incorporated herein by reference in theirentirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The present invention was made with support from the United StatesGovernment under Grant number NR0000-11-C-0145 awarded by the UnitedStates Government. The United States Government has certain rights inthe invention.

FIELD

The present invention relates to infrared detectors. More particularly,the present invention relates to a passivation of infrared detectorsusing oxide layer.

BACKGROUND

The performance of infrared Focal Plane Arrays (FPAs) is determined bythe signal to noise ratio of the photon detectors. Two parameters thatmay control the sensitivity are the noise of the detectors withoutillumination (dark noise) and the dynamic range of the Read OutIntegrated Circuit (ROIC)/sensor hybrid. Both parameters are determinedby the dark current of the devices, that is to say the current generatedby the sensors without illumination.

n-C-B-n devices are described in detail in U.S. application Ser. No.13/152,896 “Compound-Barrier Infrared Photodetector”, filed on Jun. 3,2011, which is incorporated herein by reference in its entirety. Duringthe manufacturing of n-C-B-n device 100, the material has to be etchedin order to electrically insulate the different pixels of the array asshown in FIG. 1 . This exposes the barrier layer 130 and can lead tohigh surface currents that can increase the overall dark current byseveral orders of magnitude. To improve the performance, the n-CB-nInAsSb devices known in the art are cleaned with a 10 s Buffered OxideEtch (BOE) followed by a 20 s water rinse prior to passivation to removeany oxidation formed on the Al-based barrier layer 130. FIG. 1 depictsn-C-B-n device 100 without the oxide layer. As shown in FIG. 1 , thedevice 100 comprises n-type contacts 110, 120 disposed above Al barrierlayer 130 which is in tum disposed above an n-type absorber layer 140.The device 100 further comprises passivation layer 150 above thecontacts 110, 120 and the barrier layer 130.

Similar devices have been fabricated using an n-B-n design based on anInAs absorber as described in an article by S. Maimon and G. W. Wicks,App. Phys. Lett 89, 151109 (2006), which is incorporated herein byreference in its entirety. In this case, the authors considered that thebarrier layer acted as a passivation layer. While this may be true, thebarrier is not protected from further processing steps that includehybridization and underfill of FPAs.

Embodiments disclosed in the present disclosure overcome the limitationsof the prior art.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts a device known in the art.

FIG. 2 depicts an embodiment according to the present disclosure.

FIGS. 3 a-f depict another embodiment according to the presentdisclosure.

FIG. 4 depicts measurements for an embodiment according to the presentdisclosure.

FIG. 5 depicts variation of the Noise equivalent Input (NEI) of an FPAfor different irradiances and dark current densities.

In the following description, like reference numbers are used toidentify like elements. Furthermore, the drawings are intended toillustrate major features of exemplary embodiments in a diagrammaticmanner. The drawings are not intended to depict every feature of everyimplementation nor relative dimensions of the depicted elements, and arenot drawn to scale.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth toclearly describe various specific embodiments disclosed herein. Oneskilled in the art, however, will understand that the presently claimedinvention may be practiced without all of the specific details discussedbelow. In other instances, well known features have not been describedso as not to obscure the invention.

As known in the art, n-CB-n InAsSb devices are cleaned with a 10 sBuffered Oxide Etch (BOE) followed by a 20 s water rinse prior topassivation to remove any oxidation formed on the Al-based barrier. Oneskilled in the art considers the native oxide layer as an inappropriatelayer to terminate the atomic bonds of the barrier before SiO₂encapsulation and believes that oxidation build-up on the barrier layeris detrimental to the electric performance of the n-CB-n devices.Contrary to the prior art, presently disclosed embodiments demonstratethat oxidation does not increase the dark current and oxidation may actas a good initial passivation layer before encapsulation with otherdielectric layers such as SiO₂.

As described below, in some embodiments, using native or artificiallygrown oxide layers to prepare the surface of the devices for passivationterminates the atomic bonds of the barrier layer, acting itself aspassivation layer prior to encapsulation. This reduces the dark currentof the detectors by about three orders of magnitude compared to theprevious surface treatment. By reducing the dark current, presentlydisclosed processes enable imaging at much lower photon fluxes (photonstarved conditions). Also, by reducing the dark current, it is possibleto operate the detectors at higher temperatures. This decreases theweight, volume and power consumption of infrared camera systems.

Referring to FIG. 2 , in one embodiment presently disclosed, an infrareddetector 200 comprises an absorber layer 210 supporting a barrier layer220. In some embodiments, the absorber layer 210 comprises an n-typematerial. In some embodiments, the absorber layer 210 comprisesInAs_(x)Sb_(1-x) material. In some embodiments, the barrier layer 220comprises AlGaAsSb or AlAsSb material. The infrared detector 200 furthercomprises one or more contact structures 230, 235 disposed above thebarrier layer 220. The infrared detector 200 also comprises apassivation layer 240 above the one or more contact structures 230, 235and the barrier layer 220. The infrared detector 200 further comprisesan oxide layer 245 disposed between the passivation layer 240 and thebarrier layer 220.

In some embodiments, an infrared detector 300 is formed by forming anabsorber layer 320 above a substrate 310 as shown in FIG. 3 a ; forminga barrier layer 330 above the absorber layer 320 as shown in FIG. 3 b ;forming a contact layer 340 above the barrier layer 330 as shown in FIG.3 c ; dry etching the contact layer 340 to form the one or more contactstructures 345, 350 and exposing one or more portions 360 of the barrierlayer 330 that were previously covered by the contact layer 340 as shownin FIG. 3 d ; forming an oxidation layer 365 to cover the one or moreportions 360 of the barrier layer 330 as shown in FIG. 3 e . In someembodiments, the oxidation layer 365 is formed using, for example, an O₂plasma process, a thermal oxidation process, and/or natural oxidationprocess where the infrared detector 300 is allowed to sit at a roomtemperature for a pre-determined period of time. In one embodiment, theoxidation process creates a passivation layer composed of Al-oxides,As-oxides and Sb-oxides when the barrier layer 330 comprises AlAsSbmaterial. These oxides act as a passivation layer that perfectlyterminates the dangling atomic bonds generated by the dry etchingprocess used to form the one or more contact structures 345, 350. In oneembodiment, the oxidation layer 365 is formed on the sides of the one ormore contact structures 345, 350.

In some embodiments, at least a portion of the one or more contactstructures 345, 350 and the oxide layer 365 are encapsulated in SiO₂layer 370 (shown in FIG. 3 f ) deposited by, for example,Plasma-enhanced Chemical Vapor Deposition (PECVD) process. The SiO₂layer 370 prevents the oxidation layer 365 from being removed or alteredby further processing of the infrared detector 300.

In some embodiments, the above described semiconductor layers are formedby a molecular beam epitaxy (MBE) process or any other process known inthe art.

Infrared detector samples A-E were prepared and tested to demonstrateeffectiveness of the presently described embodiments. The samples weredry etched down to the barrier layer and different surface preparationsand passivations were applied for different samples as described below.

Sample A: After etching down to the barrier layer, the sample was etchedfor 10 s in BOE and rinsed for 20 s in water. It was then passivatedusing PECVD SiO₂. This is a reference sample, corresponding to aninfrared detector known in the art and shown in FIG. 1 .

Sample B: After etching down to the barrier layer, the sample wasdirectly passivated using SiO₂ deposited with an e-beam evaporator.

Sample C: After etching down to the barrier layer, the sample wasoxidized for 2 min in an O2 plasma generated with a power of 100 W. Itwas then passivated using PECVD SiO₂.

Sample D: After etching down to the barrier layer, the sample wasoxidized for min in air on a hot plate at 100 C. It was then passivatedusing PECVD SiO₂.

Sample E: After etching down to the barrier layer, the sample wasoxidized for 2 min in an O2 plasma generated with a power of 100 W. Itwas then passivated using SiO₂ deposited with an e-beam evaporator.

Finally, windows were opened into the passivation layer and ohmiccontacts were formed. The dark current and tum-on voltage were measuredat 120K to determine any improvement in performance and the results areshown in Table 1.

TABLE 1 Dark current Turn-on density Sample Surface TreatmentPassivation V A/cm2 A BOE etch SiO₂ PECVD 0.2 1.40E−06 B None SiO₂e-beam 0.4 1.00E−08 C 2 min 02 plasma SiO₂ PECVD 0.6 1.00E−09 D 10 minon hot plate SiO₂ PECVD 0.3 1.00E−09 E 2 min 02 plasma SiO₂ e-beam 0.52.00E−09

Comparing sample A and B, it is clear that the BOE etch increases thedark current of the devices. This is because the BOE solution removesthe thermal oxide that naturally grows on top of the barrier layerbetween the dry etching and passivation steps. The absence of oxideincreases the surface leakage of the devices.

Sample C, D and E were intentionally oxidized just after etching. Thisimproved the dark current by at least one order of magnitude. Thetechniques used to grow the oxide did not impact the performance of thedevices at operating bias. Similar dark current performances wereachieved at operating bias.

However, the techniques used to grow the oxide did impact theperformance of the devices at the tum-on voltage. As shown in Table 1,tum-on voltage of Sample D is lower compared to Samples C and E. This isadvantageous for FPA fabrication because the ROIC applies up to about500 mV of bias. Therefore, the oxidation process used for sample D maybe used for the fabrication of very low dark current FPAs based onn-CB-n devices. FIG. 4 depicts current/voltage measurements of SampleDat 120K.

As shown, the oxidation of the barrier layer improved the dark currentof n-CB-n devices by 3 orders of magnitude at operating bias. Such areduction in dark current increases the sensitivity of FPAs at very lowirradiance. FIG. 5 describes the variation of the Noise equivalent Input(NEI) of an FPA for different irradiances and dark current densities.The NEI is a standard measure of the sensitivity of the infrared camera.The dotline corresponds to the Background Limited Performance (BLIP),which is the lowest theoretical NEI that can be achieved. According tothe simulation, a detector with a dark current density of 10⁻⁶ A·cm⁻²achieves BLIP for background fluxes superior to 10¹³ ph·s⁻¹·cm⁻². Incomparison, a detector with a dark current density of 10 −9 A·cm⁻²achieves BLIP for background fluxes superior to 10¹⁰ ph·s⁻¹ A·cm⁻². Dueto reduction in dark current achieved with this process, FPAs are beable to perform imaging at much lower photon fluxes.

More details about n-C-B-n devices that are not disclosed above can befound in

-   U.S. application Ser. No. 13/152,896 “Compound-Barrier Infrared    Photodetector”, filed on Jun. 3, 2011, which is incorporated herein    by reference in its entirety.

While several illustrative embodiments of the invention have been shownand described, numerous variations and alternative embodiments willoccur to those skilled in the art. Such variations and alternativeembodiments are contemplated, and can be made without departing from thescope of the invention as defined in the appended claims.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contentclearly dictates otherwise. The term “plurality” includes two or morereferents unless the content clearly dictates otherwise. Unless definedotherwise, all technical and scientific terms used herein have the samemeaning as commonly understood by one of ordinary skill in the art towhich the disclosure pertains.

The foregoing detailed description of exemplary and preferredembodiments is presented for purposes of illustration and disclosure inaccordance with the requirements of the law. It is not intended to beexhaustive nor to limit the invention to the precise form(s) described,but only to enable others skilled in the art to understand how theinvention may be suited for a particular use or implementation. Thepossibility of modifications and variations will be apparent topractitioners skilled in the art. No limitation is intended by thedescription of exemplary embodiments which may have included tolerances,feature dimensions, specific operating conditions, engineeringspecifications, or the like, and which may vary between implementationsor with changes to the state of the art, and no limitation should beimplied therefrom. Applicant has made this disclosure with respect tothe current state of the art, but also contemplates advancements andthat adaptations in the future may take into consideration of thoseadvancements, namely in accordance with the then current state of theart. It is intended that the scope of the invention be defined by theClaims as written and equivalents as applicable. Reference to a claimelement in the singular is not intended to mean “one and only one”unless explicitly so stated. Moreover, no element, component, nor methodor process step in this disclosure is intended to be dedicated to thepublic regardless of whether the element, component, or step isexplicitly recited in the claims. No claim element herein is to beconstrued under the provisions of 35 U.S.C. Sec. 112, sixth paragraph,unless the element is expressly recited using the phrase “means for . .. ” and no method or process step herein is to be construed under thoseprovisions unless the step, or steps, are expressly recited using thephrase “step(s) for.”

What is claimed is:
 1. A method of manufacturing an infrared detector,the method comprising: forming an absorber layer responsive to infraredlight; forming a barrier layer on the absorber layer; forming aplurality of contact structures on the barrier layer; forming an oxidelayer having a first portion above the barrier layer and between theplurality of contact structures and a second portion extending from thefirst portion along sides of the plurality of contact structures facingone another, and forming a passivation layer above the oxide layer, thepassivation layer covering at least a portion of a top surface of acontact structure of the plurality of contact structures and defining anopening therethrough that exposes a portion of the top surface of thecontact structure.
 2. The method of claim 1, wherein the forming theoxide layer implements an O₂ plasma process.
 3. The method of claim 1,wherein the forming the oxide layer implements a thermal oxidationprocess.
 4. The method of claim 1, wherein the forming the oxide layerimplements a natural oxidation process, and wherein the infrareddetector is allowed to sit at a pre-determined temperature for apre-determined period of time.
 5. The method of claim 4, wherein thepre-determined temperature is room temperature.
 6. The method of claim1, wherein the oxide layer comprises Al-oxides.
 7. The method of claim1, wherein the oxide layer comprises As-oxides.
 8. The method of claim1, wherein the oxide layer comprises Sb-oxides.
 9. The method of claim5, wherein the forming the oxide layer comprises covering at least aportion of one contact structure out of the plurality of the contactstructures.
 10. The method of claim 1, wherein the forming thepassivation layer implements a Plasma-enhanced Chemical Vapor Depositionprocess.